For various applications with demanding synchronization constraints, for example the synchronization of base stations of a mobile network, methods for the distribution of a reference time and/or a reference frequency on the packet-switching networks are being developed. For example, a Network Time Protocol (NTP) work group of the IETF is developing an upgrade to the NTP protocol initially specified in RFC 1305. The Precision Time Protocol (PTP) of the IEEE has been revised with this in mind. The ITU-T has defined a physical layer technology for the distribution of a reference frequency on an Ethernet network, called Synchronous Ethernet and described in the specifications G.8261, G.8262 and G.8264.5.
The IEEE 1588V2 protocol or Precision Time Protocol release 2 (PTPV2) is being studied for supporting the distribution of time and frequency in the context of stringent applications such as mobile networks. Accuracy requirements in wireless networks are about 50 ppb for frequency and about one microsecond for time. In the context of Packet-Switched Networks (PSNs) and Time distribution, the performance of the IEEE 1588V2 protocol is mainly limited by the packet jitter and the communication path delay asymmetry, both often referred to as “network noise”. The former is related to Packet Delay Variations (PDVs). The latter is the result of the difference between the communication delay of one PTPV2 message in one direction (e.g. from Master to Slave) as compared to the delay of a related PTPV2 message with the same sequence number in the opposite direction (e.g. from Slave to Master).
PTPV2 performance is very dependent on the PSN background traffic level, which is unpredictable. In order to remove/control these dependencies, Transparent Clock (TC) and Boundary Clocks—(BC) hardware support have been introduced in the PTPV2 standard.
In “Time and phase Sync noise budget in G.8271”, Stefano Ruffini, ITU-Drafts, 2010, every node of a network has a Boundary Clock and/or a Transparent Clock.
“Synchronization in next-generation mobile backhaul networks” by A. Magee, IEEE Communications magazine vol. 48, no. 10, 2010 describes a network comprising a grandmaster clock, intermediate nodes in which are placed boundary and transparent clocks and ordinary clocks. It is recommended that all intermediate nodes can work in either a boundary clock or transparent clock to overcome the effects of packet delay variation.
“Synchronizing PTPv1 and PVPv2 clients with one common time source” by H. Gerstung, International IEEE Symposium on Precision clock Synchronization for Measurement, Control and Communication, 2008 describes a Hirschmann MICE-20 Industrial Ethernet Switch which comprises two units: one in boundary clock mode with two IEEE 1588-2002 modules and one with two IEEE 1588-2008 modules which can be configured to operate in boundary clock mode or 1-step and 2-step transparent clock mode.